The default encoding for ruby scripts is now UTF-8 [#6679]. Some people report
that it affects existing programs, such as a some benchmark programs that have
become very slow .
The spec may change for the official release.

iconv was removed. It was deprecated for ruby 1.9.x. You should use
the M17N features instead like String#encode.

There is ABI breakage . We think that normal
users can/should just reinstall extension libraries.
You should be just aware "DO NOT COPY .so OR .bundle FILES FROM 1.9".

There are some comparatively small incompatibilities.
Please report any other incompatibility you notice.

FYI: We are creating upgrade notes, but some parts are already outdated.